This invention relates generally to electric motor driven submersible pumping systems and, more particularly, to methods and apparatus for controlling operation of a submersible pump.
Known deep well, residential service, submersible pumps typically are driven with two pole, alternating current (AC) induction motors packaged for immersion in a well. The motors include a stator portion that is encapsulated with an epoxy to form a barrier impervious to moisture. The motor is enclosed in a housing assembly having water lubricated bearings. The enclosed assembly is filled with ethylene-glycol. An output shaft of the motor is directly coupled to a shaft of a pump that includes a stack of impellers to force water into an outlet pipe. The outlet pipe has a pressure level determined by the depth of the water level and the pressure level at the associated residence. A check valve in the pump outlet pipe prevents water from draining into the well when the pump outlet pressure is less than the pressure in the outlet pipe.
Motors for residential water pumping systems are typically rated at 3/4 horsepower, have a 1.6 service factor, and thus have a net continuous rating of 1.2 horsepower. The motor and pump are coupled in line and typically fit into an outer casing four inches in diameter. The casing assembly has a total length of about three to four feet. Wiring and a supply pipe are attached to the pump and motor assembly before the pump and motor assembly are lowered into the well. The assembly is positioned a short distance from the bottom of the well to avoid sand and other contaminants from fouling the water inlet. Maximum operating depth can be up to 400 feet and the pump capacity is preferably sufficient to maintain 60 psi plus the pressure needed to overcome the up to 400 foot head.
The pumping system at the top of the well includes a storage tank with a spring loaded or air initiated bladder to minimize the change in pressure when the water level in the tank drops due to use by the residence. A pressure switch with adjustable hysteresis is interfaced to the storage tank to switch the pump "ON" when the pressure drops below a minimum set point and "OFF" when the pressure reaches a maximum set point.
The four inch pump-motor diameter requires a five inch well casing, which results in a substantial well drilling cost. In addition, if a well is pumped dry, the pump may be damaged because the bearings are water lubricated, and the lack of water leads to bearing failure unless a flow restrictor is added to the waterline at the well head to prevent the output flow from exceeding the well recovery rate. Further, sand, stone chips, or other debris in the well may cause the pump to seize or bind leading to a stalled motor condition that may cause motor overheating and damage. Still further, if line voltage is low, the motor is forced to operate at less than rated magnetic flux, thus requiring more current to produce the same torque, which may lead to motor overheating and the possibility of eventual failure. Also, use of an integrated gate bi-polar transistor pulse width modulation inverter as an induction motor drive may have a high output of electromagnetic interference. In addition, failure of the pump-motor results in an interruption of the potable water supply.
An AC induction motor typically has a pullout torque (maximum torque on the motor characteristic curve) which is 3 to 4 times the rated torque and a typical current at stall which is 5 to 6 times the rated current. In an application where the motor is started by simply connecting it across the power source using a switch or contactor, there is an initial inrush current of 5 to 6 times the rated current which gradually reduces to rated current as the motor accelerates to rated speed. During the acceleration, the torque increases with increased speed until the pullout torque speed is reached, after which the torque and current begin to fall as the speed increases further. The speed will settle to a constant value when the motor torque is equal to the load torque.
Torque loads presented to the motor by pumps and other variable speed loads, such as compressors and fans, vary with shaft speed. With these types of loads, the load torque at zero speed is very small and increases with increasing speed. The torque available to accelerate the load is the difference between the motor torque and the load torque. The ideal fan torque characteristic is a torque which varies with the square of speed. Pumps and compressors are oftentimes similar to the fan load torque, but in some instances may depart significantly from the ideal characteristics due to variations in back pressure, for example. In general, torque can be considered to be a function of slip frequency where a linear approximation has sufficient accuracy for most applications. If motor speed is known from a tachometer or other speed measuring device, then the controller, to produce a desired level of torque at that speed, calculates the frequency that would place the synchronous speed at the rotor speed and then adds to that frequency the slip frequency needed to produce the desired torque. For example, if the motor is running at 1800 rpm, 30 Hz excitation would make this the synchronous speed for a two pole motor. Typically, a slip frequency of 3 Hz provides 200% of rated torque so that providing 33 Hz excitation at this speed will result in 200% torque. This principle of control is usually referred to as slip control and is well known in the art.
In highly competitive markets, a tachometer or other speed sensor adds too much cost to a controller, and systems are built without speed sensing apparatus. A motor without speed sensing apparatus should change speed slowly to ensure that the motor continues to operate at slip frequencies equal to or less than the frequency corresponding to pullout torque. When the frequency source is an electronic unit where the maximum current determines the controller cost, the maximum current limit is typically set at about twice the required continuous current rating by cost constraints. If the frequency is allowed to increase significantly faster than the motor speed, the system may get into a state where the slip frequency is so high that the current limit causes the maximum torque developed to be significantly less than rated torque causing the motor to stall. If there is no speed measuring device, there may be no way for the controller to recognize that a stall has occurred and current will continue to be supplied at the limit value causing the motor to overheat and be damaged. While the description of this concern was based upon increasing the frequency too fast, the same state may arise as the result of load torque impulses, sticky shafts, and other anomalies that cause the motor shaft speed to drop.
Accordingly, it would be desirable to provide a motor that monitors the current flowing to the motor and adjusts the current in accordance with the present operating conditions. It also would be desirable to reduce the electromagnetic interference caused by the motor assembly and the controller. Further, it would be desirable to reduce the failures of the motor due to the motor becoming jammed with rocks and debris.